1. Field of the Invention
The present invention relates to a backlight unit (BLU) for a flat display, and more particularly to a backlight unit using a light guide panel (LGP).
2. Description of the Related Art
Generally, a flat liquid crystal display (LCD) is a non-emissive type display which is not self-luminous, and as such, it requires an external light source. In contrast, a self-emissive type display, such as a plasma display panel (PDP), a field emission display (FED), or the like, does not require an external light source. The non-emissive type display thus needs a backlight unit which constantly illuminates the whole display surface.
FIG. 1 is a side view illustrating a conventional backlight unit. As shown, the backlight unit 100 includes a reflecting sheet 140, a light source 130, a light guide panel 110, first and second diffusing sheets 150 and 180, and first and second prism sheets 160 and 170. In the figure, a Z-axis is parallel with the illuminating direction of the backlight unit 100 (i.e. a normal line to an upper face 114 of the light guide panel 110), an X-axis is parallel with the advancing direction of light emitted from the light source 130, and a Y-axis is perpendicular to the X and Z axes.
The light guide panel 110 has upper and lower faces 114 and 112 opposite to each other, and first and second side faces 116 and 118 opposite to each other. The light source 130 is positioned exactly outside the first side face 116 of the light guide panel 110, and emits light toward the first side face 116. The light guide panel 110 guides light, which is introduced into the light guide panel via the first side face 116, to the second side face 118 by means of internal reflection. The light guide panel 110 is provided, on an undersurface of the lower face 112, with a plurality of randomly-arranged dot patterns 120. The respective dot patterns 120 have a shape of a hemispherical groove so as to reflect and diffuse incident light. That is, the respective dot patterns 120 break a total internal reflection condition at a boundary between the light guide panel 110 and external air space so as to transmit light, reflected and diffused from the respective dot pattern 120, through the upper face 114 of the light guide panel 110. When examining luminance distribution shown on the upper face 114 of the light guide panel 110, the luminance at a viewing angle of 0° is low, and the luminance at a larger viewing angle is high. Here, the viewing angle of 0° represents the case where an observer views the light guide panel in the direction parallel with the Z-axis. In the present invention, the diffusion means scattered reflection (or scattering) at a non-optical surface, mirror reflection at a non-planar surface, and so forth.
The reflection sheet 140 is arranged such that an upper face thereof faces or confronts the lower face 112 of the light guide panel 110, so as to reflect light, which is transmitted through the dot patterns 120 of the lower face 112 of the light guide panel 110, toward the inside of the light guide panel 110.
The first diffusing sheet 150 is arranged such that a lower face thereof confronts the upper face 114 of the light guide panel 110, so as to scatter and transmit the incident light. The first and second diffusing sheets 150 and 180 each scatter the incident light so as to disperse the luminance distribution, which has been concentrated at a high viewing angle, toward the low viewing angle.
The first prism sheet 160 is arranged such that a lower face thereof confronts the upper face of the first diffusing sheet 150, and consists of a base film 162 and a plurality of prism mountains 164 protruding from an upper face of the base film 162 in such a way as to be spaced parallel with each other. Here, the respective prism mountains 164 extend parallel with the X-axis (i.e. parallel with the normal line of the first side face 116 of the light guide panel 110). The first prism sheet 160 collects, transmits, and reflects incident light on a cross sectional thereof (i.e., a Y-Z plane, or a plane perpendicular to its longitudinal direction). The first and second prism sheets 160 and 170 each serve to concentrate light on a low viewing angle, at which luminance distribution is low.
The second prism sheet 170 is arranged such that a lower face thereof confronts the upper face of the first prism sheet 160, and consists of a base film 172 and a plurality of prism mountains 174 protruding from an upper face of the base film 172 in such a way as to be spaced parallel with each other. Here, the respective prism mountains 174 extend parallel with the Y-axis (i.e. perpendicular to the normal line of the first side face 116 of the light guide panel 110). The second prism sheet 170 collects, transmits, and reflects incident light on a cross sectional thereof (i.e., an X-Z plane, or a plane perpendicular to its longitudinal direction).
The second diffusing sheet 180 is arranged such that a lower face thereof confronts the upper face of the second prism sheet 170, so as to scatter and transmit incident light.
The conventional backlight unit 100 as described above has drawbacks as follows.
First, since the backlight unit 100 has to use two high-cost prism sheets 160 and 170, thus the manufacturing cost is higher, and the thickness of the sheets make the backlight unit undesirably thicker. Further, in a case of using a prism sheet 160 or 170, the luminance of the backlight unit 100 is greatly reduced by approximately half.
Second, the multi-reflection of light generated between the first and second prism sheets 160 and 170 may cause the defect of appearance, such as a Moire interference fringe.
Third, since the backlight unit 100 has to use at least one of the diffusing sheets 150 and 180, the problem arises in that the manufacturing cost and the thickness thereof come higher and thicker, respectively. Further, the luminance of the backlight unit 100 can be also reduced by approximately 15% when using two diffusing sheets 150 and 180.